I think the policy is that the seats have to have in date cartridges if considered live and useable. Cartridges have a life of 3 years, are classified as firearms and are very expensive. This leads to the deactivation of seats - typical in JPs. However, a deactivated seat may not be as inactive as supposed as a JP incident at North Weald some years ago showed. The seat wasn't secured properly, but this wasn't evidednt to the pilot as he assumed the seat was safe and didn't do the checks normally required of a live seat. When inverted the seat moved, some of the supposedly deactivated automatics worked as advertised and a passenger was pulled out of the aircraft by the parachute which deployed from the seat while still in the aircraft. The passenger survived, but was very lucky as he hadn't done the harness up properly.
A flying school in the States I once visited had a 'deactivated seat' which lots of people including kids played with. I noticed that the guillotine was still armed. The owner was fairly shocked by the force of the thing going off when I trigggered it with a biro to make it safe.
Ejector seats are bl**dy dangerous. If you get a chance to fly an aircraft with one fitted, ask about it and learn - even if it is 'deactivated'.